CONSERVATION OF ARCHAEOLOGICAL IRON: CONSERVATION, PLANNING AND THE RETRIEVAL OF ARCHAEOLOGICAL...

LES DOSSIERSDE L’IPW15

Institut du Patrimoine wallon

La conservation-restaurationdes métaux archéologiques :des premiers soins à la conservation durable

Sébastien Clerbois (dir.),

Cristel CappuCCi, Joëlle Moulin, Letizia nonne, Sylvia pierMarini, Marie-Hélène sChuMaCher (éd.)

La c

onse

rvat

ion-

rest

aura

tion

des

mét

aux

arch

éolo

giqu

es : d

es p

rem

iers

soi

ns à

la c

onse

rvat

ion

dura

bleLe traitement de conservation-restauration et la mise en valeur patrimoniale des métaux

archéologiques sont des questions complexes. Le développement de l’archéologie, notamment préventive, oblige les administrateurs en charge des fouilles à gérer une quantité de plus en plus grande de matériel. Avec le développement des métiers du patrimoine, le nombre d’intervenants, de la fouille au musée, est de plus en plus grand, et ces acteurs sont de plus en plus spécialisés. En ce qui concerne la conservation-restauration, les méthodes évoluent vite. Du travail mécanique au plasma, les moyens d’interventions ne manquent pas, mais la pertinence est encore débattue au niveau international. Autre problème : le long de la chaîne opératoire, les acteurs n’ont pas souvent l’occasion de faire converger leurs approches pour optimiser l’étude et la conservation. Mise en état de l’objet pour étude, stabilisation, consolidation, conservation passive et/ou restauration, comment aborder ces questions collectivement pour définir au mieux les gestes techniques, et en particulier dans les contextes d’urgence et de budgets réduits de l’archéologie préventive ? L’ambition de cet ouvrage vise à enrichir les pratiques de chaque discipline sur base d’un dialogue interdisciplinaire et à stimuler la pratique de l’intervention concertée de manière à rassembler les gestes techniques dans ce qui serait un protocole de traitement global, bien défini, du sol à la vitrine de musée.

Prix de vente : 25 €ISBN : 978-2-87522-152-0

La conservation-restauration des métaux archéologiques :

des premiers soins à la conservation durable

La conservation-restauration des métaux archéologiques :

des premiers soins à la conservation durable

Actes du colloque organisé par la Direction de l’Archéologie du Service public de Wallonie et la DGO4, la FAW (Fédération des Archéologues

de Wallonie et de Bruxelles) et le CReA-Patrimoine (ULB), à l’Université libre de Bruxelles, salle Dupréel, les 8 et 9 mai 2014.

Sébastien Clerbois (dir.)Cristel CappuCCi, Joëlle Moulin, Letizia nonne, Sylvia pierMarini,

Marie-Hélène sChuMaCher (éd.)

LES DOSSIERSDE L’IPW, 15

5

TABLE DES MATIÈRESREMERCIEMENTS ___________________________________________________________________________________________________________________________ 11

AVEC LE SOUTIEN DE _____________________________________________________________________________________________________________________ 11

PRÉFACE J. Plumier _________________________________________________________________________________________________________________________ 13

LES MÉTAUX, ÉPISTÉMOLOGIE D’UN DOSSIER SENSIBLE DE LA CONSERVATION-RESTAURATION DU PATRIMOINE ARCHÉOLOGIQUE C. CaPPuCCi et al. ________________________________________________________________________ 15

THE MONITORING OF ARCHAEOLOGICAL METALS, FROM EXCAVATIONS TO MUSEUM: SOME ISSUES AND PERSPECTIVES R. Bertholon _____________________________________________________________________________ 19

TransmiTTing informaTion _____________________________________________________________________________________________________________ 20Developing communicaTion Tools beTween professionals __________________________________________________________________ 20increasing number of arTefacTs ____________________________________________________________________________________________________ 21from prevenTive To preDicTive conservaTion ___________________________________________________________________________________ 22DocumenTing inTangible conDiTion as well as maTerial conDiTion _______________________________________________________ 22locaTing The limiT of The original surface _____________________________________________________________________________________ 23imaging Techniques ______________________________________________________________________________________________________________________ 24fuTure works _____________________________________________________________________________________________________________________________ 24acknowleDgmenTs ________________________________________________________________________________________________________________________ 25

01. La préservation des métaux archéologiques dès la fouille : protocoles de prélèvement et techniques de conservation sur les chantiers archéologiques Archaeological Metals Taking and Preservation from the Excavations:

Sample Techniques and Preservation Methods on the Excavation Fields

FROM EXCAVATION TO PRESERVATION: PREVENTIVE CONSERVATION APPROACHES IN ARCHAEOLOGICAL BRONZE COLLECTIONS O. oudBashi ____________________________________________________________________ 29

inTroDucTion _______________________________________________________________________________________________________________________________ 29

corrosion of archaeological bronzes: a shorT DescripTion __________________________________________________________ 29

main risk facTors ______________________________________________________________________________________________________________________ 30

prevenTive conservaTion in archaeological bronze collecTions _____________________________________________________ 31

pre-excavaTion consiDeraTions ____________________________________________________________________________________________________ 31

During-excavaTion acTiviTies _______________________________________________________________________________________________________ 32

long-Term preservaTion ______________________________________________________________________________________________________________ 32

inTegraTing prevenTive conservaTion: The global approach ____________________________________________________________ 33

conclusion _________________________________________________________________________________________________________________________________ 35

acknowleDgemenTs ______________________________________________________________________________________________________________________ 35

LA CONSERVATION PRÉVENTIVE DES MÉTAUX IN SITU, DIALOGUE ENTRE L’ARCHÉOLOGUE DE TERRAIN ET LE SPÉCIALISTE EN CONSERVATION-RESTAURATION ? C. Proust _____________________________ 37

La conservation-restauration des métaux archéologiques :des premiers soins à la conservation durable

6

02. L’apport des études métallographiques pour la conservation-restauration des métaux anciens The Contribution of Archaeometallurgic Studies for Preservation and Restauration of Ancien Metals

PATINES NATURELLES DES BRONZES – QUELQUES DÉVELOPPEMENTS MAJEURS L. roBBiola ___________ 45inTroDucTion _______________________________________________________________________________________________________________________________ 45

a. paTine proTecTrice Des bronzes en milieu naTurel _______________________________________________________________________ 47A. 1. Cas général et décuprification _____________________________________________________________________________________ 47A. 2. Altération des patines protectrices : variations morphologiques ___________________________________ 48

b. paTine « vile » eT corrosion imporTanTe ___________________________________________________________________________________ 49

conclusion _________________________________________________________________________________________________________________________________ 50

OBJETS FERREUX : APPROCHES ARCHÉOMÉTRIQUES P. dillmann ____________________________________________________ 51inTroDucTion _______________________________________________________________________________________________________________________________ 51

pourquoi analyser : comprenDre la place Du fer Dans les sociéTés anciennes ____________________________________ 51

commenT analyser : aDapTer les méThoDes aux quesTionnemenTs hisToriques _______________________________________ 55

conclusion _________________________________________________________________________________________________________________________________ 57

COMPOSITION AND CORROSION FORMS ON ARCHAEOLOGICAL AND NON-ARCHAEOLOGICAL HISTORIC PRINTING LETTERS FROM THE MORAVIAN MUSEUM, MEMORIAL OF KRALICE BIBLE, THE CZECH REPUBLIC AND THE MUSEUM PLANTIN-MORETUS ANTWERP, BELGIUM P. storme et al. ____________________ 59

inTroDucTion _______________________________________________________________________________________________________________________________ 59

The museum planTin-moreTus ______________________________________________________________________________________________________ 59

The memorial of kralice _____________________________________________________________________________________________________________ 60

1. research objecTives ________________________________________________________________________________________________________________ 61

2. experimenTal __________________________________________________________________________________________________________________________ 612. 1. Analysis of the printing letter stored in the Museum Plantin-Moretus (Antwerp) ___________ 612. 2. Analysis of the printing letters in the Kralice Memorial of Brno _____________________________________ 62

3. resulTs __________________________________________________________________________________________________________________________________ 623. 1. Museum Plantin-Moretus types ____________________________________________________________________________________ 62

3. 1. 1. Composition __________________________________________________________________________________________________________ 623. 1. 2. Corrosion ______________________________________________________________________________________________________________ 633. 1. 3. Relation composition-corrosion ________________________________________________________________________________ 63

3. 2. Kralice Memorial types _________________________________________________________________________________________________ 633. 2. 1. Composition __________________________________________________________________________________________________________ 633. 2. 2. Experiment description ___________________________________________________________________________________________ 643. 2. 3. Corrosion ______________________________________________________________________________________________________________ 643. 2. 4. Relation composition-corrosion ________________________________________________________________________________ 64

4. conclusion ____________________________________________________________________________________________________________________________ 65

Table des matières


7

LA PRISE EN COMPTE DES SURFACES ET PATINES ANTIQUES DANS LA RESTAURATION DES MÉTAUX ARCHÉOLOGIQUES : ÉTUDE DE LA PLAQUE À L’AURIGE D. roBCis et al. ________________________ 67

inTroDucTion _______________________________________________________________________________________________________________________________ 67

la plaque à l’aurige, présenTaTion eT examen général _____________________________________________________________________ 68

raDiographie eT émissiographie De rayons x ___________________________________________________________________________________ 70

analyses élémenTaires __________________________________________________________________________________________________________________ 70

DiffracTion De rayons x ______________________________________________________________________________________________________________ 72

Discussion _________________________________________________________________________________________________________________________________ 73

conclusion _________________________________________________________________________________________________________________________________ 74

03. Les techniques de restauration : une mise en perspective critique The Restauration Techniques: a Critical Approach

CONSERVE ALL, RESTORE NOTHING? CHANGING ATTITUDES IN METAL CONSERVATION B. sChmutzler ____ 79inTroDucTion _______________________________________________________________________________________________________________________________ 79

Terms anD DefiniTions __________________________________________________________________________________________________________________ 80

eThics of conservaTion anD resToraTion ________________________________________________________________________________________ 80

The role of conservaTion science ________________________________________________________________________________________________ 82

research projecT saving from rusT (sfr) ___________________________________________________________________________________ 82

DigiTal conservaTion ___________________________________________________________________________________________________________________ 84

conclusion _________________________________________________________________________________________________________________________________ 85

acknowleDgmenTs ________________________________________________________________________________________________________________________ 85

COMPARISON OF CONSERVATION METHODS APPLIED TO IRON ARTEFACTS K. sChmidt-ott ______________ 87inTroDucTion _______________________________________________________________________________________________________________________________ 87

iron corrosion ___________________________________________________________________________________________________________________________ 87

early conservaTion TreaTmenTs ______________________________________________________________________________________________________ 88

conservaTion aT The swiss naTional museum __________________________________________________________________________________ 89

DesalinaTion in alkaline sulphiTe ___________________________________________________________________________________________________ 89

prevenTive conservaTion ______________________________________________________________________________________________________________ 91

special challenges ______________________________________________________________________________________________________________________ 91

Table des matières


8

CONSERVATION OF ARCHAEOLOGICAL IRON: CONSERVATION, PLANNING AND THE RETRIEVAL OF ARCHAEOLOGICAL INFORMATION – A DIFFERENTIATED APPROACH N. Cleeren _______________________________ 93

inTroDucTion _______________________________________________________________________________________________________________________________ 93

The conservaTion process ___________________________________________________________________________________________________________ 941. Conservation as a means of gathering of information ______________________________________________________ 94

1. 1. Four levels of information ___________________________________________________________________________________________ 941. 2. Common Practice – diagnostic and investigative cleaning ______________________________________________ 951. 3. Optimal retrieval of information? _________________________________________________________________________________ 951. 4. Other non-invasive techniques in search for more detailed information ____________________________ 961. 5. Cost-effectiveness of different approaches ___________________________________________________________________ 98

2. Conserving this information in its physical form ______________________________________________________________ 983. Presentation ____________________________________________________________________________________________________________________ 99

conclusion _________________________________________________________________________________________________________________________________ 99

acknowleDgmenTs ________________________________________________________________________________________________________________________ 99

04. Musée et conservation préventive Museums and Sustainable Preservation

ENVIRONNEMENT MUSÉOGRAPHIQUE : QUEL IMPACT SUR LES OBJETS MÉTALLIQUES ? P. de ViViés ______ 103première éTuDe De cas : les maTériaux en alliage cuivreux... ou apparenTés _____________________________________ 103

seconDe éTuDe De cas : les maTériaux en plomb _____________________________________________________________________________ 105

Troisième éTuDe De cas : les objeTs en argenT _______________________________________________________________________________ 106

quaTrième éTuDe De cas : les objeTs en fer ___________________________________________________________________________________ 106

LES POLLUANTS DANS LES COLLECTIONS ET LES MATÉRIAUX POUR LA CONSERVATION. DÉTECTION ET QUANTIFICATION PAR VOLTAMMÉTRIE, EXEMPLES D’APPLICATION F. urBan __________ 109

quels carTons uTiliser pour conserver les DaguerréoTypes ? ________________________________________________________ 110

Du sable en fonD De viTrine ? _____________________________________________________________________________________________________ 111

le rôle De la poussière ______________________________________________________________________________________________________________ 111

conclusion _______________________________________________________________________________________________________________________________ 112

LONG-TERM METALS PRESERVATION IN ARCHAEOLOGICAL STORAGES AND MUSEUMS M. duBus et al. ____ 1131. inTroDucTion ________________________________________________________________________________________________________________________ 113

2. experimenTal ________________________________________________________________________________________________________________________ 114

3. resulTs ________________________________________________________________________________________________________________________________ 114

4. conclusions _________________________________________________________________________________________________________________________ 115

5. acknowleDgmenTs __________________________________________________________________________________________________________________ 116

Table des matières


9

05. Vers une plateforme des bonnes pratiques. Gestion des réserves et conservation durable Towards a Platform of Good Practices. Managing the Museum Reserves for Long-term Preservation

LE CENTRE DES COLLECTIONS DU MUSÉE NATIONAL SUISSE ET L’ENTREPOSAGE DES OBJETS DE COLLECTION B. sChüle _________________________________________________________________________________________ 121

affolTern am albis ___________________________________________________________________________________________________________________ 121

l’enTreposage Des objeTs De collecTion ________________________________________________________________________________________ 122

conclusion _______________________________________________________________________________________________________________________________ 125

LE MOBILIER ARCHÉOLOGIQUE MÉTALLIQUE DANS UN CHANTIER DES COLLECTIONS S. Païn ________ 127inTroDucTion _____________________________________________________________________________________________________________________________ 127

1. concevoir un chanTier Des collecTions ___________________________________________________________________________________ 127

2. la prise en compTe De la conservaTion _____________________________________________________________________________________ 1282. 1. Conservation préventive _____________________________________________________________________________________________ 1282. 2. Conservation curative et restauration ________________________________________________________________________ 128

3. la Dimension DocumenTaire eT l’accessibiliTé ____________________________________________________________________________ 129

conclusion. le chanTier pour aujourD’hui eT pour Demain _____________________________________________________________ 130

LA PRÉSERVATION DES OBJETS MÉTALLIQUES AU PAIR : LES PROCÉDURES ET LEUR ADAPTATION S. milBled et al. ______________________________________________________________ 133

1. le parcours De l’objeT méTallique : les DifférenTes éTapes eT les procéDures _____________________________ 1331.1. Prélèvement ________________________________________________________________________________________________________________ 1331.2. Première phase d’inventaire _______________________________________________________________________________________ 1341.3. Premières interventions d’urgence et / ou séchage contrôlé ________________________________________ 1351.4. Deuxième phase d’inventaire ______________________________________________________________________________________ 1351.5. Sélection des objets pour traitement pour étude et / ou radiographie X _______________________ 1351.6. Étude et précision des choix de traitement __________________________________________________________________ 1361.7. Contrôle et rangement ________________________________________________________________________________________________ 1371.8. Sélection pour conservation ________________________________________________________________________________________ 1371.9. Sélection pour valorisation __________________________________________________________________________________________ 1381.10. Veille sanitaire __________________________________________________________________________________________________________ 138

2. facTeurs influanT sur les choix De prise en charge __________________________________________________________________ 1382.1. La considération de l’objet __________________________________________________________________________________________ 1382.2. Les difficultés de conservation et les adaptations _______________________________________________________ 1392.3. Le statut juridique de l’objet et responsabilités ___________________________________________________________ 141

conclusion _______________________________________________________________________________________________________________________________ 142

remerciemenTs __________________________________________________________________________________________________________________________ 142

Table des matières


10

LA MISE EN PLACE D’UN OUTIL POUR LA CONSERVATION À LONG TERME DES COLLECTIONS ARCHÉOLOGIQUES DU SERVICE PUBLIC DE WALLONIE M.-H. sChumaCher ______________________________________ 143

genèse Du projeT _______________________________________________________________________________________________________________________ 143

le projeT prenD forme _______________________________________________________________________________________________________________ 144

zoom sur la prise en charge parTiculière Des objeTs méTalliques ___________________________________________________ 145

conclusion eT perspecTives ________________________________________________________________________________________________________ 146

LISTE DES AUTEURS _____________________________________________________________________________________________________________________ 149

BIBLIOGRAPHIE ___________________________________________________________________________________________________________________________ 153


93

CONSERVATION OF ARCHAEOLOGICAL IRON: CONSERVATION, PLANNING AND THE RETRIEVAL OF ARCHAEOLOGICAL INFORMATION – A DIFFERENTIATED APPROACH

Natalie Cleeren

The precarious condition in which archaeological iron finds are currently being excavated, presents a great challenge towards long-term preservation.As archaeologists are primarily in search for infor-mation, conservators are still often more occupied with the physical condition of these finds.Joining both approaches will result in the conser-vation (in its physical form or not) of all the infor-mation and research potential of these finds.Conservation should be a well considered process in which several steps contribute to the most rele-vant and feasible treatment option for every object.Currently, different invasive and non-invasive tech-niques for the conservation of heavily corroded archaeological iron finds are being used in order

to withdraw as much information as possible from these extremely fragile finds.The effectiveness of different conservation ap-proaches for iron objects, from standard con-ventional X-ray radiography, in combination with partial and full cleaning, to Computer To-mography (CT) is evaluated in regard to the relevancy of the information they reveal. While planning a conservation procedure, four levels of information will be considered in every step of the conservation process. As expected, a well thought-over balance between common meth-ods and new techniques will result in the most (cost) effective approach towards cleaning and preservation of these finds.

inTroDucTionDuring the last ten years, iron finds from several Iron Age, Roman and Early Medieval sites in Bel-gium and The Netherlands (The sites of Blicquy (Iron Age & Roman), Broechem (Merovingian), Viesville (Merovingian) in Belgium and the site of Lent (Merovingian) in Nijmegen, the Netherlands) were subjected to investigative conservation tech-niques and their research potential was constantly examined and re-examined. The majority of these objects (spearheads, arrowheads, axes, swords, knives, tools…) are heavily corroded with little or no metal core left. However, their original shape is conserved within the corrosion layers. This con-dition gives rise to substantial difficulties when

trying to reach the original surface by mechanical cleaning, also referred to as the «abandonment surface» by Bertholon (2007, 34), which suggests a representation of the dimensions of the object at the moment when it was interred.

A sensible approach to the conservation of such heavily corroded iron artefacts is required, as a majority of iron finds are currently being recov-ered in this precarious condition. In a context of privatization of archaeological research, where the retrieval of information gathering is always urgent and financial resources are limited, a realistic ap-proach imposes itself. Four levels of information


94Les techniques de restauration : une mise en perspective critique

The Restauration Techniques: a Critical Approach

were established in close collaboration with the archaeologists involved in the research of the mentioned sites. Even elaborate techniques such as µ-CT, can become part of a cost-effective ap-proach when information retrieval is set forward as the main objective of archaeological iron finds conservation in the first stages of the archaeolog-

ical research. The evaluation of the effectiveness of passive and active stabilisation techniques be-comes increasingly important as every intervention needs to be accounted for. Extensive interventions in relation to the presentation of such objects can only be justified after careful selection, based on a well considered process of information gathering.

The conservaTion processA protocol for the conservation treatment of iron finds should consider not only the condition of the finds and their conservation needs, but most importantly, their archaeological context and the potential of the objects to reveal additional in-formation through careful selection of treatment method. A treatment protocol can be estab-

lished for every type of archaeological context depending on specific research questions and the overall research potential of an individual object.Economic reality imposes a careful selection in cleaning methods, stabilisation methods and con-servation treatments for presentation purposes.

1. Conservation as a means of gathering of information

1.1. Four levels of informationThe objective of archaeological conservation could be described as efforts to safeguard those characteristics of excavated objects that make them informative, awe-inspiring or enjoyable (pye 2001). During the initial phases of archaeolog-ical research, iron finds serve purely as sources of archaeological information. In this stage, the conservator’s focus lies mostly in recovering all information residing in the object, with a large fo-cus on diagnostic features that place the find in a typological sequence.During the conservation of iron finds, first and fore-most to uncover the archaeological information they contain, it becomes useful to distinguish different levels of information that are directly connected to the priorities established by the archaeologists.The first and most relevant level of information in-cludes the diagnostic features, such as the «orig-inal» shape and possible decoration, placing the object in a well known typology. The detailed iden-tification of organic materials also belongs to this first category (specifically in regard to early Medie-val and earlier finds) as they reveal the scarce in-

formation on the use of organics that has survived burial.These include traces of the initial burial environ-ment, traces of associated organic finds and or-ganic materials belonging to the object itself. Their recovery has become increasingly significant to archaeologists.The second level of information is specific detail of shape and decoration that have the potential to become part of the commonly used set of diagnos-tic features. So far these features are not included in current typological sequences for iron objects, mainly because they have been overlooked and are too difficult to document using common clean-ing and 2D documentation techniques. Refining the existing typological sequences remains a prior-ity in archaeological research.The third level contains mostly technological infor-mation, of great interest to the study of technolog-ical evolution. Details of metal-working are unfor-tunately seldom incorporated into archaeological studies. This information is currently difficult to link to date or origin of the finds, due to a lack of in-depth research and is considered more anec-dotal that the information from levels 1 and 2.




Metallurgical studies unfortunately recede into the background as typological studies will more easily reveal economic and social evolution.The last and fourth level of information deals with the corrosion features of the object and should become increasingly relevant to archaeologists when dealing with the management of archaeo-logical sites. In situ preservation has been set out as the preferred option in the Valetta Convention (1992) but unfortunately, research on the correla-tion between soil contamination and the condition of metal artefacts is lacking, certainly in Belgium. Fundamental studies on this topic are lacking for this region, although these could have substantial consequences for in situ preservation of archaeo-logical sites. In situ preservation might turn out not to be the best option for long–term preservation of metal, and specifically iron, objects.

1.2. Common Practice – diagnostic and investigative cleaning

The first step in every treatment protocol should be the focus on obtaining level 1 information, to be gathered by partial (diagnostic and investigative) cleaning, in close collaboration with archaeologists and finds specialists. This process will form the basis for the conservation assessment in view of further interventions.Extracting a maximum of information from a large number of objects has become the main priority of archaeological conservation. As an example, in 2001, the Flemish Institute for the Archaeological Heritage determined that a period of 24 years was required to fully clean approximately 25.000 iron finds from the site of the Roman Castrum (camp) of Oudenburg, Belgium. This was subsequently nar-rowed down to 2,5 years by X-raying every object and partial cleaning. In total, 213 objects were fully cleaned for presentation purposes (Cleeren 2006). From this moment on, partial (diagnostic and inves-tigative) cleaning of iron artefacts was introduced in Belgium as common practice. Diagnostic cleaning provides the archaeologists with sufficient informa-tion on the morphological details to place an object in its typological sequence. Investigative cleaning provides details, such as the high-valued traces of organics (both level 1 information).

The 12-year experience with partial and investiga-tive cleaning has shown that the first premise for successful partial cleaning is a close collaboration between conservators, archaeologists, finds spe-cialists and especially draughtsmen. It demands intensive knowledge of typological features by the conservator, who preferably has a strong back-ground in archaeology. One of the biggest chal-lenges is to teach draughtsmen to interpret X-ray images. Ideally, the conservator produces the first diagnostic drawings as shown in figure 33.

1.3. Optimal retrieval of information?It has to be realised that partial cleaning, together with the information retrieved from two–dimen-sional X-rays will never reveal every important morphological detail. As an example, the X-ray of a spearhead from Viesville (Belgium) surprisingly does not show the small copper alloy rim at the bottom of the shaft. Fortunately this area was sub-ject to partial cleaning and this feature was discov-ered, however, localised elsewhere, it would have been missed (figure 34).

Figure 33.

Roman object from the site « ville d’Anderlecht » in Blicquy, Belgium (Fouilles 2003 CTRA et ULB). X-ray, zones to be partially cleaned (yellow boxes) and end result (archaeological drawing).© Natalie Cleeren

Figure 34.

End of the shaft from a spear-head with copper alloy rim, not visible on the X-ray (site of Viesville / Pont-à-Celles, Belgium).© Laurence baty

SPW-Archéologie




Traces of organic materials can be missed by par-tial cleaning. 2D X-rays are restricted to the metal object, hardly showing any less-dense materials. The more an organic compound has mineralised, the more it might show up on the X-ray. Partial cleaning will only reveal them when visible on the X-ray, when localised in the diagnostic areas to be cleaned or of course when they are (partly) visible by the naked eye.How accurate are the data obtained by diagnostic cleaning? Usually, the original contours of an object will be more evident on the X-ray than on the actu-al object. 181 objects from the sites of Broechem and Viesville, Belgium (swords, spearheads, arrow-heads, axes) were examined after partial cleaning, registering the deviation between the measure-ments of the cleaned areas and the measurements taken by X-ray (scaled to precisely 1:1) (Cleeren, dhaene and peters, 2013). Most drawings (and the archaeological record) are based on the X-rays and are generally quite accurate. Set against the diag-nostic features that categorise these objects, a de-viation of 1-3 mm plays no role in their archaeolog-ical interpretation. Depending on the type of object, even an exceptional (3% of the analysed objects) 5 mm deviation can sometimes be acceptable (for level 1 information, larger objects).

Even with extensive, full cleaning, specific mor-phological (level 1 and 2) details can be over-looked. During previous research, special attention was given to the morphology of iron spearheads (Cleeren, dhaene and peters 2013). Here the lim-its of our current approach were reached: classic X-rays do not show the section of the blade. Is it for example diamond-shaped or lenticular? Mechani-cal cleaning, however meticulously executed, will often fail to provide the correct shape of these sections on very heavily corroded objects. Even experienced conservators are reluctant to admit that not every morphological detail, not every de-tail of the «abandonment surface», can be recov-ered and even more reluctant to admit they may, at some point, unconsciously proceed to «shape» the object according to their own assumptions.

Revealing every informative feature of an archae-ological iron object will often require full cleaning.

Given the time and budgetary constraints, full cleaning must be warranted and based on the in-formation obtained from partial cleaning, and with reference to the importance of the archaeological context. This is not a decision, made by the con-servator alone.

1.4. Other non-invasive techniques in search for more detailed information

In search for more information, several objects from the Merovingian site of Lent (Nijmegen, Neth-erlands) have been examined by X-ray computed tomography (CT) to obtain a 3-D representation of the objects without any cleaning (Cleeren, dhaene and peters 2013). Several ensembles of block-lift-ed grave goods were investigated, still encased in their block of soil (using the HECTOR scanner) (MassChaele, dieriCk, van loo e.a. 2012) at the Ghent University Center of Computed Tomography (UGCT). Relating this technique to the different levels of information addressed in this paper was the scope of the investigations by µ-CT of the block lifted objects form Lent. More specifically, which level 1 information can be retrieved by µ-CT alone and not by full cleaning, and which information remains difficult or impossible to retrieve using µ-CT?A µ-CT-scan of a spearhead blade (site of Lent), executed earlier at the University of Leuven reveals a section evolving from a lenticular to diamond shape (and back), a shape would be difficult for a conservator to determine visually by mechanical cleaning (figure 35b). Based upon the information gathered from partial cleaning (figure 35a), many conservators may be tempted to «shape» the en-tire blade in a lenticular shape. This level 2 infor-mation is not incorporated in any typologies for Merovingian spearheads, not even in the ones that do incorporate data on the section of the blade (siegMund 1998).Visualizing sections of the object proves to be use-ful for its interpretation and recovery of level 2 in-formation.Internal structures, technological details, objects encased in their carrier (such as a scabbard) re-main concealed during hands on cleaning but can be beautifully revealed using µ-CT.




A scabbard made from organic material with im-portant morphological detail is not easily recovered by mechanical cleaning. Not every surface detail is visible even when meticulously cleaning using a binocular microscope (40 x magnif.) (figure 36).µ-CT scans of ex situ organic samples can provide detailed images of their internal structure, which facilitate exact identification (level 1 information) (haneCa, deforCe, boone e.a. 2012). The most im-portant drawback of non-invasive documentation by µ-CT (XCT and NCT) for block lifted finds, is the fact that organic materials may be localised when partly or wholly mineralised, but cannot be exam-ined in full detail for purposes of exact material identification through a block of soil. Investigative or full cleaning of the ensemble remains neces-sary to fully reveal detailed and level 1 information about associated organics. For example, the tiny string attaching a small yellow bead to the han-dle of a sword was completely missed by the µ-CT scan (figure 37).

CT scanning can reveal morphological details, sometimes impossible to uncover by mechanical cleaning. However, these visualisation techniques alone cannot provide all the information archae-ologists require from these finds, least of all for block-lifted objects.

Figure 35.

a) Morphological information gathered from diagnostic cleaning. b) Morphological information obtained by µCT scan (spearhead, Merovingian burial site of Lent (Netherlands).© BAMN (Bureau Archeologie

& Monumenten, gemeente/ Nijmegen). Natalie Cleeren

Figure 36.

µ-CT scan image of the organic scabbard with surface details (Sword, Lent, BAMN, Bureau Archeologie & Monumenten, gemeente / Nijmegen) 175 KV, 50 W, 1000 ms, voxel size : 50 µm.© Jelle dhaene, UGent

Figure 37.

Yellow bead with string preserved. The string is not visible on the µ-CT scan (SWORD, LENT, BAMN (Bureau Archeologie & Monumenten, gemeente / Nijmegen).© M. peters & Jelle dhaene




1.5. Cost-effectiveness of different approaches

Cost-effectiveness of the different approaches discussed here, has been evaluated. For individ-ual finds, the combination of 2D X-rays with di-agnostic cleaning remains the most cost-effective method for retrieving as much level 1 information as possible. The expense of meticulous diagnostic cleaning is in average a third of the cost of full me-chanical cleaning (considered for both large and small finds, calculated for the finds of Broechem, Blicquy and Viesville). Some details might be missed and small deviations in shape will occur, but these do not justify the expense of fully clean-ing every iron artefact. Microscopic investigation of organic materials remains more cost-effective than the current use of µ-CT for isolated organics.For object ensembles « en bloc », the most cost-ef-

fective and scientifically interesting approach seems to be a combination of 2D X-rays and well chosen µ-CT scans to obtain information that can-not be retrieved by hand, and as a guidance tool for investigative cleaning of these complex ensem-bles. For the block-lifted objects from Lent (Ni-jmegen), this approach will be equally expensive as full cleaning for blocks containing swords and several small utensils). Full cleaning will reveal an object, ready for presentation and might reveal some additional details such as the string going through the yellow bead mentioned above, while partial and well chosen investigative cleaning in combination with µ-CT, will reveal a greater set of morphological (level 1) information.Communication is the main key to a successful and budgetary responsible conservation process, and the main objective is the retrieval of all scien-tific information from the objects.

2. Conserving this information in its physical formAfter exploiting these objects to their fullest extent in terms of the information they reveal, the next step in the conservation process will be to actually conserve the objects holding this information.Active stabilisation of archaeological iron can be fully separated from the cleaning process. Al-though many types of objects will be easier to clean after stabilisation (this possibility can be included in the conservation plan), not every partially or ful-ly cleaned object should necessarily, immediately receive an extensive active stabilisation treatment or even be stored in the most optimal conditions.Ideally, every iron object is fully cleaned, fully sta-bilised, and stored in the most favourable condi-tions. Meanwhile this ideal has proven to be un-achievable. Therefore, numerous efforts are being undertaken to establish more feasible and better preventive measures for storing iron artefacts as well as differentiated approaches to active stabi-lization techniques (Watkinson and riMMer 2013 ; Matthiesen and steMann-petersen 2013). Neverthe-less, experience shows that many iron objects will actively corrode to a sever extent when no active stabilisation treatment had been performed.

Can we, as conservators, allow archaeological iron objects to be stored in less than perfect condi-tions? Can we even consider disposing of certain sets of objects, a subject at hand but controversial in archeology? Selection as a concept is being ac-cepted, but how to actually make the selection re-mains a difficult issue and can only be performed in close collaboration with archaeologists.

It seems favourable to spend a little more time on thorough risk assessments of iron finds that will result in sensible, differentiated approaches to-ward the conservation of iron, then trying to treat every object extensively and in the same manner to maximise their chances of survival. Conserva-tors need to realise they are conserving witnesses from a distant past, stories that lie within these objects and their potential to reveal even more. They also have to realise that in archaeology, the better story is told after analysing the informa-tion derived from all finds of an archaeological context, and not when only the information from a partial set of extensively conserved objects is available.




3. PresentationTo present these finds to the general public, further, more detailed conservation treatments may be undertaken, not by definition prioritis-ing the aesthetic qualities of the object as was usually the case in the recent past. Here again, when these objects are used to tell a story, their informative value should be highlighted, even when conflicting with the commonly accepted

(or presumed) aesthetic appearance. The public is still often confronted with black, shiny objects, the result of inappropriate, past treatments with epoxies, polyesters… Nevertheless, the public can be educated to look at, enjoy and interpret objects that show their archaeological identity, if necessary, assisted by modern visualisation techniques.

conclusionEconomic reality no longer allows conservators to spend too much time on treatments that will not provide more scientific information. Archaeologi-cal objects are primarily being preserved in view of the knowledge they can provide.2D radiography combined with partial cleaning can provide a method that reveals a lot of infor-mation in a limited time-span. The information that remains uncovered by this method can be safeguarded for future research. Often, more intensive investigation is considered unfeasible. Nevertheless, in-depth studies using a combina-tion of invasive (partial cleaning) and non-inva-sive techniques such as µ-CT can be achieved when working in close collaboration with archae-

ologists, following a well though-out protocol that considers the retrieval of, and need for, different levels of information throughout the conservation process.Although it can be interesting to rigidly separate the different steps in the conservation process, in order to analyse their efficiently, these steps are an integral part of a general conservation plan for archaeological iron objects, to be set forward and evaluated for every individual site.Overall a differentiated approach towards conser-vation and a close collaboration, from site to mu-seum or depot between conservators and archae-ologists will results in the best scientific data set, and the best story.

acknowleDgmenTsThe X-ray computed tomography image of the spearhead has been made on the KU Leuven CT facilities, financed by the Hercules Founda-tion (project AKUL 09/001). Collaboration with the research group «Materials Performance

and Non-Destructive Testing» of Prof. M. Wev-ers, and the technical assistance of Ing. J. Vanhulst, at the Department of Metallurgy and Materials Engineering KU Leuven, is greatly ap-preciated.

CONSERVATION OF ARCHAEOLOGICAL IRON: CONSERVATION, PLANNING AND THE RETRIEVAL OF ARCHAEOLOGICAL...

Documents

Transcript of CONSERVATION OF ARCHAEOLOGICAL IRON: CONSERVATION, PLANNING AND THE RETRIEVAL OF ARCHAEOLOGICAL...